A : Coordination number of both `Na^(+)` and `Cl^(-)` NaCl is 6.
R : Second coordination number of `Cl^(-)` in the NaCl unit is 12.

Options

A. If both Assertion & Reason are true and the reason is the correct explanation of the assertion.
B. If both Assertion & Reason are true but the reason is not the correct explanation of the assertion.
C. If Assertion is true statement but Reason is false.
D. If both Assertion and Reason are false statements.

Answer

To solve the question, we need to analyze both the assertion and the reason provided regarding the coordination numbers of Na⁺ and Cl⁻ in NaCl.

### Step-by-Step Solution:

1. **Understanding Coordination Number**:
- The coordination number of an ion in a crystal lattice refers to the number of nearest neighboring ions surrounding it.

2. **Analyzing the Structure of NaCl**:
- Sodium chloride (NaCl) crystallizes in a face-centered cubic (FCC) structure.
- In this structure, Cl⁻ ions occupy the corners and face centers of the cube, while Na⁺ ions occupy the octahedral voids.

3. **Determining the Coordination Number of Cl⁻**:
- Each Cl⁻ ion is surrounded by 6 Na⁺ ions.
- Therefore, the coordination number of Cl⁻ is 6.

4. **Determining the Coordination Number of Na⁺**:
- Each Na⁺ ion is surrounded by 6 Cl⁻ ions.
- Thus, the coordination number of Na⁺ is also 6.

5. **Evaluating the Assertion (A)**:
- The assertion states that the coordination number of both Na⁺ and Cl⁻ in NaCl is 6. This is true based on our analysis.

6. **Evaluating the Reason (R)**:
- The reason states that the second coordination number of Cl⁻ in the NaCl unit is 12.
- The second coordination number refers to the number of ions surrounding a given ion after the first layer of coordination.
- In the case of Cl⁻, after the first layer of 6 Na⁺ ions, there are indeed 12 additional Na⁺ ions surrounding it, leading to a second coordination number of 12.

7. **Conclusion**:
- Both the assertion and the reason are true.
- However, the reason does not correctly explain the assertion since the assertion is specifically about the first coordination number, while the reason discusses the second coordination number.

### Final Answer:
Both assertion (A) and reason (R) are true, but R is not the correct explanation of A.